Many wells that produce oil and/or gas have sufficient internal pressure to flow the well fluid to the surface. The well has a casing and typically a string of tubing that extends downward through the casing. The well fluid flows up the tubing to a production tree at the surface. The tubing is supported by a tubing hanger that lands in the wellhead assembly.
For various reasons, the tubing must be pulled from time to time for remedial operations. Heavy liquid can be circulated into the well to overcome the internal formation pressure before pulling the tubing. However, in some instances killing the well in this manner can cause damage to the formation. In another procedure, rather than killing the well, the tubing is pulled and re-run while the well is still under pressure. The operator typically employs a snubbing unit to trip the tubing while under pressure.
A typical snubbing unit assembly operates with a blowout preventer (“BOP”) that has an annular element that will seal around the tubing while it is being pulled or lowered. Often the pressure in a well may be sufficiently to push the tubing upward through the BOP, particularly when most of the tubing has been pulled from the well. The snubbing unit has at least one set of stationary slips that grip the tubing to prevent upward movement of the tubing and at least a set of traveling slips that grip the tubing to push it downward. Hydraulic cylinders stroke the traveling slips to push the tubing string downward while the blowout preventer is closed around the tubing.
Some snubbing units are constructed to operate independently of a workover rig. These stand alone units mount on top of a BOP to both pull and run the tubing. A weight supporting set of stationary and traveling slips are mounted to the unit for pulling the tubing. These stand alone units may also include a rotary mechanism for performing certain drilling and milling operations. In these units, a tubing guide may be employed to prevent buckling of the tubing.
Other snubbing units are constructed to operate in conjunction with a workover rig, and are referred to as rig assist units. A workover rig has a blowout preventer and a set of elevators and stationary slips to pull the tubing. However, a workover rig does not normally have snubbing slips to prevent upward movement of the tubing while pulling under pressure. Also, a workover rig does not normally have the ability to push the tubing into the well under high pressure. The rig assist unit has traveling and stationary snubbing slips and hydraulic cylinders that will accomplish these tasks. Because of space requirements and the lack of need, a rig assist unit would not have a number of items that a stand alone unit would have.
Service companies that provide snubbing units or jacks often have both stand-alone and rig assist snubbing units for the different customer needs. Maintaining both types of units adds expense and causes scheduling problems.
The snubbing units, whether rig assist or stand alone, normally have four hydraulic cylinders of the same diameter or pressure area. These cylinders will be rated to supply a selected amount of force. Deeper wells or wells with higher pressure may require higher capacity cylinders. Generally, the hydraulic pump and its associated components are matched to the capacity of the cylinders. A very high capacity jack will not be required on shallower wells and wells with lower pressure. Furthermore a very high capacity jack may be physically too large for smaller tasks or the stroking rate may be too slow. Consequently, an operator may have jacks of different capacities to match different customer needs. Additional sizes of jacks adds to inventory costs and create scheduling problems.